Thermally sprayed coatings are used in many industrial applications in various sectors: aerospace, automobile, energy, biomedical, pulp and paper, etc. The role of the deposited coatings is generally to protect a part from corrosion, erosion, abrasion or high temperature. Coatings can also be used to promote the biocompatibility of the part, to create a functional surface or to build new parts or repair worn ones. In these applications, for various reasons it is desirable to determine the thickness of the applied coatings.
Thermal spray processes are difficult to control on the production floor. Many factors drift from their optimum value, and small deviations in conditions may result in considerable variation in the deposition. For example, the properties of a feed of powders (or other deposition materials) in a hot gas jet (plasma or flame) are crucial as they influence directly the trajectories of the powers in the gas jet, influencing the powder temperature and velocity and thus the coating structure, and deposition efficiency. Also, in DC plasma spray processes, the wear of the electrode with time due to the high intensity current (300-1000 A) in the torch affects the plasma properties, and consequently also the powder conditions and the coating structure and deposition efficiency.
Variations in deposition efficiency lead to variation in the resulting coating thickness deposited in a certain period of time. This is a major concern in the industry. Today, control of the final coating thickness is often done by measuring it directly by comparing the thickness of the coated part with its thickness before deposition. The measurements are carried out using mechanical gauges such as micrometers, vernier calipers, etc. Other methods such as eddy current or magnetic induction are also used when the magnetic properties of the coating and/or substrate materials make this possible. These approaches are time consuming as they require the part to be cooled down to room temperature before the measurement is carried out manually by the operator. If the coating thickness is below the target thickness, the part has to be returned in the spray room, reheated and coated again to reach the desired thickness. If the coating is too thick, the coating has to be stripped and the coating must be sprayed again.
As the control of the coating thickness is currently done after spraying due to the limitations of the measurement techniques, there is a tremendous need for a measurement technique that can be used on-line during spraying. If such a technique were available, the operator would have the possibility to stop the process when the coating thickness is within the desired thickness window for the application.
Moreover, in some industrial applications, very large parts must be coated requiring many days of spraying. During this period of time the deposition efficiency may drift due to reasons like those mentioned above. For example, the deposition efficiency may drift by up to 20% in plasma spray processes over the time required to coat a surface. In such cases, the time to deposit the desired thickness and the quantity of powder used increases by 20% negatively affecting the productivity of the process.
The techniques mentioned above (mechanical gauges, eddy current or magnetic gauges) that are currently used in the industry can not be used during spraying as the part is hot, located in an hostile spray environment and generally in movement. Furthermore testing based on electromagnetic properties is not suitable for all deposition materials, and would require expensive equipment to reliably serve in such an environment.
There are other techniques that can be used to measure the coating thickness of sprayed coatings that have cooled. Many approaches use ultrasonic techniques in various configurations. For example, Carslon et al. (U.S. Pat. No. 6,363,787) take advantage of the surface waves to measure the thickness of metallic coatings. Laser-ultrasonic techniques can be used also as they don't require a direct contact with the coated parts. However, the cost for implementing such a technique in production limits strongly its market. Furthermore, these ultrasonic techniques generally require an a priori knowledge of the elastic properties of the coatings in order to evaluate their thickness. Unfortunately these properties are generally not known for thermal spray coatings and they depend strongly on the spray parameters, temperature of the substrate, etc.
Nuclear radioactive techniques can be used also to monitor the thickness of coatings as taught by P. Cielo, in Optical Techniques for Industrial Inspection, Academic Press, Inc., San Diego, 1988, pp. 47-53. One of the main advantages of these techniques is that they are non contact techniques. However, they require adequate safety shielding to protect human bodies from the harmful radiation. An example of this technique for measuring the thickness of paint deposited on a metal substrate is described by MacKenzie (U.S. Pat. No. 6,252,930). X-ray tomography can also be employed to measure the coating thickness but it is difficult to accomplish during spraying as the sensing equipment is quite large. Moreover, the cost for such equipment limits considerably the use of this approach.
Optical techniques can be used to measure the coating thickness. Measurements can be carried out by a variety of techniques such as optical triangulation, interferential techniques or others. These techniques make it possible to determine the surface profile of the parts without contact. The coating thickness is obtained by measuring the dimensions of the part before coating deposition and during or after deposition. The coating thicknesses have been obtained from the difference between the two measurements. This approach has only been used on relatively thin parts. On large parts, the change in the dimensions of the parts due to the thermal expansion is not negligible as compared to the coating thickness. Typically the temperature of the part changes from room temperature to 100-300° C. during spraying. The resulting change in the part dimensions due to thermal expansion may reach a few millimeters, which is one order of magnitude larger that the typical thickness of thermal spray coatings. Other surface profilometry techniques (with or without contact) have also limited applicability for thermal spray coatings as they suffer the same limitations as the optical profiling techniques.
Coating thicknesses can be measured also by thermal-wave methods (see Cielo, pp. 387-389). Using these techniques, the surface of the coating is heated by a laser or another source of heat, and the evolution of the coating surface temperature is monitored. The coating thickness is deduced from the time evolution of the surface temperature knowing the thermal properties of the coating and substrate materials. Bantel et al. (U.S. Pat. No. 4,818,118) describe such a technique applied to the monitoring of the thickness of thermal barrier coatings.
In typical thermal spray conditions used in industry, the coating thickness per pass ranges typically from 10 to 25 microns (may reach 50 microns in some cases). As the coating is formed by the solidification of 10-100 micron diameter particles, its surface is relatively rough. The mean roughness value (Ra) is about 5 to 10 microns having peaks and valleys up to 20-40 micron magnitude. In such conditions, measuring the surface profile with a precision sufficient to extract the value of the coating step formed during a single pass is quite a challenge. The roughness of the coating is larger than the typical thickness deposited per pass. Moreover the substrate is, in general, not perfectly formed, making the measurement of the step (a few microns height on meter-long parts) very difficult or even impossible in certain cases. Moreover, the spraying environment on the production floor is not adapted to sophisticated detection instruments for determining the coating surface profile. There are vibrations due to the rotation of the part, movement of the torch and high throughput ventilation. The environment may be dusty because of the powders fed in the process and the fumes generated when the powders are exposed to the high temperature flame.
What is needed therefore is a method of measuring deposition thickness in real time under typical thermal spray conditions used in the industry.